conebeckham wrote:Nothing is "emptiness." Not even "emptiness." Emptiness is not a thing. It could be said that "Buddha Nature" is "empty," but to say that "all emptiness" is the same as "Buddha Nature" is not right. "The Emptiness" of a banana, for example, is not equal to "The Emptiness" of Buddha Nature. Emptiness does not exist apart from it's object...the two truths, Conventional/Relative and Absolute, are inseparably conjoined in this way.
Emptiness is generally equated to dependent origination. That which is dependently originated (pratītyasamutpāda) is empty (śūnyatā). Buddha-nature, being the capacity for buddhahood, is dependently originated meaning it occurs due to causes and conditions. Bananas likewise are dependently-originated upon causes and conditions. Both buddha-nature and bananas share the quality of being empty of inherent-nature (svabhava).
There is in Chinese a certain translation by Paramārtha called the
Buddha Nature Treatise (Fóxìng lùn 佛性論). The Sanskrit title would probably have been
Buddhadhātu-śāstra, but unfortunately it is not extant.
The key assertion of the text is that all sentient beings without exception "possess" buddha-nature and therefore have the capacity for anuttara-samyak-sambodhi which means that any teaching concerning icchantikas being forever unable to free themselves from saṃsāra is merely provisional.
The text actually does equate buddha-nature with emptiness and tathatā. Consider the following:
三為離虛妄執者。若人有此慢心。則於如理如量。正智不得生顯故起虛妄。虛妄者。是眾生過失。過失有二。一本無。二是客。一本無者。如如理中。本無人我。作人我執。此執無本。由無本執故。起無明等。由無明起業。由業起果報。如此三種。無實根本。所執是無故知能執皆成虛妄故。由於此執所起無明諸業果報。並是虛妄故。無受者作者。而於中執有是虛妄。故言本無。二是客者。有為諸法皆念念滅無停住義。則能罵所罵二無所有。但初剎那為舊。次剎那為客。能罵所罵起而即謝。是則初剎那是怨。次則非怨。以於客中、作於舊執、此執不實、故名虛妄。若起此執。正智不生。為除此執故說佛性。佛性者。即是人法二空所顯真如。由真如故。無能罵所罵。通達此理。離虛妄執。
III. Making [beings] free from delusional attachments. There may be a person who has this arrogant mind and then right wisdom, both innate and experiential, cannot manifest and so there arises delusions. These delusions are the errors of beings. There are two errors. The first is reification. The second is transience. “Reification” is where while within the principle of suchness (tathatā) there is fundamentally no self-identity, there is a delusional attachment to a self-identity. This delusional attachment is without a base. As this is a baseless delusional attachment, there arises ignorance (avidyā) and so on. Due to ignorance there arises karma. Due to karma there arises retribution (vipāka). Like this these three types are without a real faculty base. What one is attached to is non-existent and thus it is known attachments all are delusions. Due to these attachments and the arisen ignorance, karma and retribution together being delusions, there is no receiver and no doer, but therein there is the attachment and one has these delusions. Therefore we call this reification (lit. “without base”). “Transient” has the meaning that because dharmas all moment-to-moment extinguish they are without static abiding and so ridiculing and the ridiculed both do not exist. Just as the first moment (kṣaṇa) becomes the former past, the following becomes the present. Ridiculing and the ridiculed arise and then disappear. Thus in the first moment there is one's object of animosity and in the next they are not one's object of animosity. In the present this attachment, created by the former past attachment, is not real and thus it is called delusional. If one arouses this attachment, true wisdom will not be produced. In order to remove this attachment buddha-nature is explained. Buddha-nature is the suchness (tathatā) revealed through the two emptinesses of person and phenomenon. Due to suchness there is no ridiculing or ridiculed. Penetrating this principle one is free from delusional attachments.
The key statement here is, "Buddha-nature is the suchness (tathatā) revealed through the two emptinesses of person and phenomenon." Buddha-nature can be said to be the capacity for realization and attainment of Buddhahood. This capacity, dependent upon causes and conditions ergo empty, is essentially only possible by virtue of tathatā. To say a certain sentient being is without buddha-nature is the same as asserting that this sentient being has the inherit nature of being an unenlightened worldly being. However, we know this is not the case because inherent and fixed ontological natures are untenable.
Consider this further:
難曰。若汝謂我立無根眾生。有二過失者。汝立犯重一闡提人無有佛性。永不得涅槃。亦有二失。一者泰過過失。眾生本以我見無明為凡夫法。尋此無明。由違人空故起。既起無明故有業報。若不違人空。則無無明業報。既無無明業報等三輪。若爾應是聖人作於凡夫。若謂眾生無佛性者。但聖為凡。無凡得聖。此成泰過。二者不及過失。若汝謂有眾生無佛性者。既無空性。則無無明。若無無明。則無業報。既無業報。眾生豈有。故成不及。而汝謂有眾生無佛性者。是義不然。何以故。汝既不信有無根眾生。那忽信有無性眾生。以二失同故。
Criticism – If you think I am establishing [the existence] of sentient beings without faculties then you have two errors. You establish that having violated a heavy [prohibition] the icchantika has no buddha-nature and eternally will not attain nirvāṇa. Again there are two errors. The first is the error of exaggeration (atriprasaṅga). Sentient beings take self-view and ignorance (avidyā) as their worldly dharmas. Investigating this ignorance it arises due to mistaking the emptiness of the person. Having given rise to this ignorance, thus there is karma-retribution (karma-vipāka). If one did not mistake the emptiness of the person, there would then be no ignorance and no karma-retribution – as there would be no three wheels of ignorance, karma and retribution. If this is so then it should be that the holy men (ārya) are made from the worldly ones. If you think that sentient beings have no buddha-nature, then the holy are just only worldly beings and no worldly being could attain holiness. This would constitute the error of exaggeration. The second is the error of unattainability (aprāpya). If you think that there are sentient beings without buddha-nature, then as there would be no emptiness-nature [because buddha-nature is emptiness] and therefore no ignorance. If there is no ignorance, then there would be no karma and retribution – as there would be no karma and retribution, how could sentient beings exist? Thus this constitutes unattainability. However, your thinking that there are sentient beings without buddha-nature: this point is not so. Why? As you do not believe that there are beings without faculties, how could you foolishly believe that there are beings without buddha-nature as these two errors are the same.
The assertion here is that sentient beings only arise by virtue of they themselves being dependently originated. That which is dependently originated is empty. Tathatā. Buddha-nature, equated to tathatā, if utterly non-existent, would mean that sentient beings could not exist in the first place. It would further mean that sentient beings would be unable to ever at all achieve Buddhahood. However, we know neither of these to be the case. There are sentient beings and some do achieve Buddhahood.
If "Buddha Nature" were "emptiness," then why would it not just be called "emptiness?" If there are three turnings of the Wheel, then we must assume the teachings or main points of each wheel-turning are at least provisionally, or conventionally, different. There would be no sense in creating a new term, concept, metaphor, or whatever, called "Buddha Nature" as the term, concept, metaphor "emptiness" had already been introduced.
The concept of buddha-nature is basically soteriological in function.
It has to be framed in the context of the debate concerning icchantika or those who are said to forever be unable to attain liberation of any kind including arhatship.
問曰。若爾云何佛說眾生不住於性。永無般涅槃耶。
Question – if so why did the Buddha teach that [some] sentient beings do not abide in [buddha-]nature and forever are without parinirvāṇa?
答曰。若憎背大乘者。此法是一闡提因。為令眾生捨此法故。若隨一闡提因。於長時中。輪轉不滅。以是義故。經作是說。若依道理。一切眾生。皆悉本有清淨佛性。若永不得般涅槃者。無有是處。是故佛性決定本有。
Answer – If one has contempt for the Mahāyāna, this way is the cause for being an icchantika. It was in order to have sentient beings abandon this way. If one falls into the causes for being an icchantika, for a long time cyclic existence does not cease. It is because of this idea that the sūtra has this teaching. If you rely on the principle, all sentient beings entirely and fundamentally have pure buddha-nature. If [you suggest] that forever one does not attain parinirvāṇa, there is not the basis for it. For this reason buddha-nature is decidedly fundamentally present.
Basically, the idea of buddha-nature is a provisional metaphysical idea employed to demonstrate that all sentient beings have the capacity for anuttara-samyak-sambodhi.
It really is just a means to an end, but a necessary one considering that some thinkers in previous ages actively asserted that icchantikas really are forever doomed to eternal suffering in saṃsāra. To counter this one has to demonstrate that all sentient beings "have buddha-nature" which equates to them having the capacity, by virtue of emptiness, the dependently-originated process leading to anuttara-samyak-sambodhi. This process can generally be divided into three types as outlined in the following diagram:

These are specifically defined as the causal processes which may be fostered to produce buddhahood or attainment of the dharmakāya. As you can see the processes are dependent on causes which means they are dependently originated. When we say that all sentient beings have buddha-nature, we mean that all sentient beings have the capacity to initiate the aforementioned causes which lead to the attainment of the dharmakāya.
Mahayana, and in my opinion, all Buddhism, tend toward metaphysics, as they must. This is because, try as they might, they are frameworks, apparatii (?), attempting to describe something which is beyond description, don't you think?
First one must build the raft and then safely cross the river of birth and death to the other side. Any and all metaphysical ideas within Buddhism must be held as provisional and as means to an end.